1. Field of the Invention
This invention relates to a vehicle transmission, and more particularly to a vehicle transmission that includes a wet clutch installed on a crankshaft.
2. Description of the Related Art
Referring to FIG. 1, a conventional vehicle transmission 1 is adapted to transmit power from an engine of a vehicle, such as an all-terrain vehicle, a motorcycle, etc., to a rear wheel (not shown) of the vehicle. The conventional vehicle transmission 1 includes a crankshaft box unit 12 driven by the engine, a clutch unit 13 disposed in proximity to the crankshaft box unit 12, and a transmission box unit 14 disposed in proximity to the clutch unit 13.
The crankshaft box unit 12 includes a crankshaft box 121, a crankshaft 122 disposed in the crankshaft box 121 and rotatable by the engine, a driven shaft 123 having two ends connected respectively and pivotally to the crankshaft 122 and a transmission box 140 of the transmission box unit 14, a first bearing 124 disposed between the transmission box 140 and the driven shaft 123, and a self-lubricating bearing 125 sleeved on the driven shaft 123. The crankshaft 122 has two oil passages 126 that permit flow of lubricating oil therethrough.
The clutch unit 13 includes a seal cover 131 cooperating with the crankshaft box 121 of the crankshaft box unit 12 to define a clutch chamber 130, a wet clutch 132 sleeved on and rotatable with the crankshaft 122, an outer shield 133 sleeved on the crankshaft 122 for covering the wet clutch 132, a sleeve body 134 disposed between the driven shaft 123 and the outer sleeve 133 and connected fixedly to the outer shield 133 by a plurality of rivets 137 (only two are shown), a second bearing 135 disposed between the sleeve body 134 and the crankshaft 122, and a unidirectional clutch 136 for controlling unidirectional rotation of the wet clutch 132 and lubricated by the lubricating oil flowing from the oil passages 126 in the crankshaft 122. The driven shaft 123 extends through the seal cover 131, and is coupled with the sleeve body 134. When the rotational speed of the wet clutch 132 reaches a predetermined speed, the wet clutch 132 comes into frictional contact with the outer shield 133 so as to rotate an assembly of the outer shield 133, the sleeve body 134, and the driven shaft 123.
The transmission box unit 14 includes a driving pulley unit 141 sleeved on the driven shaft 123, a coupling shaft (not shown) connected fixedly to the rear wheel, a driven pulley unit (not shown) sleeved on the coupling shaft, and a V-belt 143 trained on the driving pulley unit 141 and the driven pulley unit so as to transfer rotation of the driving pulley unit 141 to the driven pulley unit. When the driven shaft 123 rotates at a speed smaller than a threshold speed, the pitch diameter of the driving pulley unit 141 is smaller than that of the driven pulley unit. In this state, the rotational speed of the rear wheel is lower than that of the driven shaft 123. When the driven shaft 123 rotates at a speed greater than the threshold speed, the pitch diameter of the driving pulley unit 141 is greater than that of the driven pulley unit. In this state, the rotational speed of the rear wheel is greater than that of the driven shaft 123.
When the engine is in an idle speed condition, although the crankshaft 122 and the wet clutch 132 rotate, the wet clutch 132 is spaced apart from the outer shield 133. As such, power cannot be transmitted from crankshaft 122 to the driven shaft 123 and, thus, the rear wheel. When a throttle is operated to increase the rotational speed of the crankshaft 122 and the wet clutch 132, the wet clutch 132 engages the outer shield 133 so as to allow for transmission of power from the crankshaft 122 to the driven shaft 123. When the engine is in an acceleration condition, the pitch diameter of the driving pulley unit 141 increases, while the pitch diameter of the driven pulley unit is reduced, thereby resulting in an increase in the rotational speed of the rear wheel. When the engine is in a deceleration condition, the pitch diameter of the driving pulley unit 141 reduces, while the pitch diameter of the driven pulley unit is increased, thereby resulting in a decrease in the rotational speed of the rear wheel.
The aforesaid conventional vehicle transmission 1 suffers from the following disadvantages:                (1) Since the crankshaft 122 and the driven shaft 123 are coupled to each other at a position between the wet clutch 132 and the outer shield 133, many elements need to cooperate with the crankshaft 122, the driven shaft 123, the wet clutch 132, and the outer shield 133 in a highly precise manner. This results in difficulties with respect to assembly of these elements.        (2) The lubricating oil flows onto the unidirectional clutch 136 and other rotating parts through the oil passages 126 in the crankshaft 122. However, a space between the unidirectional clutch 136 and the sleeve body 134 is too small to be filled with sufficient lubricating oil. This results in unsmooth rotation of the unidirectional clutch 136.        